Silicon nitride (SiN) layers (also referred to as SiN films) have become relevant in the field of large-scale integrated circuits (LSI) and other microelectronic fabrication techniques such as micro-electronic mechanical systems (MEMS). As such, selectively etching SiN layers is a step included in many state-of-the-art micro-fabrication processes. Etching selectivity is measured as a ratio of etching rates between different materials. One example etching solution used to selectively etch a SiN layer is hydrofluoric (HF) acid. However, HF acid lacks enough etching selectivity. For example, the etching rate of SiN is about 0.1 times lower than SiO2 in HF. Other approaches to SiN etching make use of organic solutions containing anhydrous HF, resulting in relatively low selectivity. Another etching solution used for SiN etching is a concentrated solution of phosphoric acid (H3PO4)—about 85% wt—in a wet bench etching tool. In a wet bench approach a group of wafers are soaked in an acid bath at a given temperature for a given time. Intrinsic drawbacks of the wet bench approach are accumulation of impurities in the acid solution, cross-contamination among wafers, as well as high defects, reducing etching performance with bath age. Also, wafer to wafer (WtW) and within wafer (WiW) etching uniformity may be compromised in wet bench procedures. Furthermore, in wet bench approaches there is the issue of etching rate drift with number of wafers processed. This is due to the impact of the concentration of silica (SiO2)—silicon oxides or silicone—in etching rate for both SiN and SiO2. For many process cycles, the concentration of silica in the bath increases steadily, leading to etching rate drifts. This phenomenon is mitigated in state-of-the-art factories by “seasoning” the wet bench using dummy Si wafers. The result is an increase in the cost and lower wafer throughput since the wet bench needs some time for “seasoning” prior to proceed with the actual wafer manufacturing. Accordingly, although existing approaches have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.
Embodiments of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.